Electrochemical fuel cells convert fuel and oxidant to electricity and reaction product. In electrochemical fuel cells employing hydrogen as the fuel and oxygen-containing gas as the oxidant, the reaction product is water. Such fuel cells generally contain a membrane electrode assembly ("MEA") consisting of a solid polymer electrolyte or ion exchange membrane disposed between two electrodes formed of porous, electrically conductive sheet material. The electrodes are typically formed of carbon fiber paper. The MEA contains a layer of catalyst at each membrane/electrode interface to induce the desired electrochemical reaction. The MEA is in turn disposed between two plates in which at least one flow passage is engraved or milled. These fluid flow field plates are typically formed of graphite. The fluid flow field passage direct the fuel and oxidant to the respective electrodes, namely, the anode on the fuel side and cathode on the oxidant side. The electrodes are electrically coupled to provide a path for conducting electrons between the electrodes.
At the anode, the fuel permeates the electrode and reacts at the catalyst layer to form cations, which migrate through the membrane to the cathode. At the cathode, the oxygen-containing gas supply reacts at the catalyst layer to form anions. The anions formed at the cathode react with the cations to form a reaction product. In electrochemical fuel cells employing hydrogen as the fuel and oxygen-containing air (or pure oxygen) as the oxidant, a catalyzed reaction at the anode produces hydrogen cations from the fuel supply. The ion exchange membrane facilitates the migration of hydrogen ions from the anode to the cathode. In addition to conducting hydrogen cations, the membrane isolates the hydrogen fuel stream from the oxidant stream comprising oxygen-containing air. At the cathode, oxygen reacts at the catalyst layer to form anions. The anions formed at the cathode react with the hydrogen ions that have crossed the membrane to form liquid water as the reaction product.
Perfluorosulfonic ion exchange membranes, such as those sold by DuPont under its Nafion trade designation, must be hydrated or saturated with water molecules for ion transport to occur. It is generally accepted that such perfluorosulfonic membranes transport cations using a "water pumping" phenomenon. Water pumping involves the transport of cations in conjunction with water molecules, resulting in a net flow of water from the anode side of the membrane to the cathode side. Thus, membranes exhibiting the water pumping phenomenon can dry out on the anode side if water transported along with hydrogen ions (protons) is not replenished. In addition, fuel cells employing such membranes require water to be removed from the cathode (oxidant) side, both as a result of the water transported across the membrane from the water pumping phenomenon and product water formed at the cathode from the reaction of hydrogen ions with oxygen.
The accumulation of water at the cathode is problematic for several reasons. First, the presence of liquid water in the vicinity of the catalyst layer reduces the accessibility of the catalyst to the reactants, resulting in a reduction in the power of the fuel cell. This phenomenon is sometimes referred to as "flooding" of the catalyst site. Secondly, the accumulation of liquid water at the cathode interferes with the permeation of reactants through the cathode to the catalyst, again resulting in a loss of power to the fuel cell. Thirdly, the accumulation of liquid water at the cathode can impart physical changes to the adjacent membrane, causing localized swelling and expansion of the membrane.
Conventional water removal techniques generally involve conducting water accumulated at the cathode away from the cathode catalyst layer and toward the oxidant stream exiting the cathode flow field plate. One conventional water removal technique is wicking, or directing the accumulated water away from the cathode using capillaries incorporated in the cathode. Another related water removal technique employs screens or meshes within the cathode to conduct water away from the catalyst layer. Still another conventional water removal technique is to incorporate hydrophobic substances, such as polytetrafluoroethylene (trade name Teflon), into the cathode sheet material to urge accumulated water away from the cathode. The conventional water removal methods can be disadvantageous because (1) conventional methods involve limited access to the catalyst site since accumulated water is removed in liquid form, and (2) the additional presence of removed water vapor in the oxidant gas stream decreases the mole fraction of oxygen in the stream.
It has been found that a new type of experimental perfluorosulfonic ion exchange membranes, sold by Dow under the trade designation XUS 13204.10, does not appear to significantly exhibit the water pumping phenomenon in connection with the transport of hydrogen ions across the membrane. Thus, the transport of water molecules across these Dow experimental membranes does not appear to be necessary for the transport of hydrogen ions as in the Nafion-type membranes. This absence of water pumping in the Dow experimental membranes avoids the accumulation of transported water at the cathode, and, more importantly, permits the transport of product water across the membrane, in a direction counter to the flow of hydrogen ions across the membrane, for removal on the anode side of the membrane electrode assembly. Water removal on the anode side can also be practiced with Nafion-type membranes. However, the degree of water pumping of such Nafion-type membranes must be considered in determining the net flux of water across the membrane.
Thus, removing water at the anode side of the fuel cell, as opposed to the cathode side, relieves flooding of the catalyst site since transported water does not accumulate in addition to product water at the cathode. Moreover, removing water at the anode side of the fuel cell permits oxygen to flow unimpeded to the cathode catalyst layer.